1. Field of the Invention
This invention relates to a process and apparatus for cyclonic combustion of fossil fuels, in particular natural gas, in a combustion chamber with cooled walls, which provides ultra-low pollutant emissions as well as high system efficiencies in watertube boilers, water heaters, and other similar devices. The combustion chamber is enveloped by a cooling fluid conduit and cooled by a cooling fluid circulating through the conduit.
2. Description of the Prior Art
Conventional combustion of fossil fuels with air produces elevated temperatures which promote complex chemical reactions between oxygen and nitrogen in the air forming various oxides of nitrogen as by-products of the combustion process. These oxides, containing nitrogen in different oxidation states, generally are grouped together under the single designation of NO.sub.x. Concern over the role of NO.sub.x and other combustion by-products, such as sulfur dioxide and carbon monoxide, in "acid rain" and other environmental problems is generating considerable interest in reducing the formation of these environmentally harmful by-products of combustion.
U.S. Pat. No. 3,934,555 discloses a cast iron modular boiler having a cylindrical combustion chamber where a mixture of gaseous fuel and air is introduced in a rotational flow around its longitudinal axis. The combustion gases are recirculated internally, thereby causing dilution of gases in the boiler. The combustion chamber is encircled by a water circulation conduit and cooled by a stream of cold water that circulates through the conduit. Heat is removed from the combustion chamber as hot water.
U.S. Pat. No. 4,714,032 teaches combustion of solid fuels charged as aqueous slurries with recirculation of condensate containing particles of ash, alkali, and spent alkali by charging such condensate to an elongated entrained phase combustion reactor. Hot, dry compressed air is injected as primary and/or secondary air into the reactor. A portion of heat liberated in the combustion zone is used to vaporize the fuel slurry water. The remainder of heat which must be absorbed to reduce the combustion temperature is extracted through a heat transfer surface in the combustion zone or absorbed by latent heat of recycled water or slurry, or by a combination of both methods.
U.S. Pat. No. 3,969,482 discloses a process for treating effluent gases containing high concentrations of sulfur oxides, nitrogen oxides, hydrogen halides, silicon tetrafluoride, and mixtures thereof. Effluent gases are treated to remove a portion of acidic gases by spraying an aqueous solution or slurry into the effluent gases.
U.S. Pat. No. 4,007,001 teaches combustion producing low NO.sub.x by tangentially introducing to a first combustion zone of 0 to 65 percent of the total air and about 5 to 25 percent of the total air to a secondary combustion zone wherein there is an orifice between the primary and secondary combustion zones. U.S. Pat. No. 3,859,786 teaches a vortex flow combustor having a restricted exit from the combustion chamber.
U.S. Pat. No. 4,021,188 and U.S. Pat. No. 3,837,788 both teach staged combustion with less than the stoichiometric amount of air in the primary combustion chamber with additional air being added to the secondary combustion chamber for completion of combustion. U.S. Pat. No. 4,575,332 teaches staged combustion in a swirl combustor with forced annular recycle of flue gas to the upstream end of the primary combustion zone.
U.S. Pat. No. 4,395,223 discloses staged combustion with excess air introduced into the primary combustion zone with additional fuel being introduced into the secondary combustion zone. U.S. Pat. No. 3,741,166 discloses a blue flame burner with recycle of combustion products with low excess air to produce low NO.sub.x while U.S. Pat. No. 4,297,093 discloses a single combustion chamber with a specific flow pattern of fuel and combustion air forming fuel-rich primary zones and fuel-lean secondary zones in the combustion chamber.
U.S. Pat. No. 4,920,925 teaches a cyclonic combustor for boilers having an uncooled primary combustion chamber, a secondary combustion chamber in communication with said primary combustion chamber, ducts for supplying fuel and combustion air directly into the primary combustion chamber and for forming a cyclonic flow pattern of hot gases for combustion within the primary and secondary combustion chanbers, an orifice disposed at the downstream end of the secondary combustion chamber, and a heat exchanger surrounding the second combustion chamber for removing heat therefrom.
U.S. Pat. No. 4,989,549 teaches a combustion apparatus for staged combustion inside the Morison tube of a firetube boiler, the first stage being substoichiometric combustion and the second stage being above-stoichiometric combustion. In accordance with one embodiment, an elongated orifice into which secondary combustion air is injected is disposed between the primary combustion zone and the secondary combustion zone and a swirler is disposed at the downstream end of the secondary combustion zone. In accordance with a second embodiment, the downstream end of the primary combustion chamber is closed off by a refractory wall and extends into the upstream end of the secondary combustion chamber. Axially disposed openings in the side wall of the primary combustion chamber permit passage of the products of combustion from the primary combustion chamber into the secondary combustion chamber. An orifice is disposed at the exit end of the secondary combustion zone through which gases from the secondary combustion chamber are exhausted.